The present invention relates generally to the art of coupling systems for connecting tubing, pipe and the like. More specifically, the present invention has application to coupling systems for connecting corrugated plastic pipe used in watertight gravity-flow drainage and sewage applications, and particularly such type of pipe having a smooth interior wall for handling increased fluid flow capacity, as well as increased internal and external hydrostatic pressures. Although the present invention has particular relevance to the above applications, it will be appreciated that the principles of the present invention may also find application in other types of pipe and tubing configurations where watertight joints are deemed desirable.
Under current standards established by the American Society of Testing and Materials (ASTM), it is required that corrugated polyethylene pipe intended for use in certain drain and gravity sewage applications achieve a watertight joint to a pressure of 10.8 psi during laboratory testing. In attempting to meet these standards, manufacturers of corrugated polyethylene pipe have traditionally utilized a bell and spigot joint design, whereby a gasket is compressed radially between the outside diameter (OD) of the spigot and the internal diameter (ID) of the bell. These types of designs have been widely utilized and are disclosed in a number of different patents (e.g. U.S. Pat. Nos. 6,126,209; 5,415,436; and 5,071,173). Some manufacturers have also attempted to utilize a means of reinforcement around the OD of the bell, as well as some stiffening foams within the internal corrugations of the sealing portion on the spigot (e.g. U.S. Pat. No. 6,578,882).
Although the traditional bell and spigot design with a radially compressed gasket does work well to provide an adequate seal in a number of applications, the design has several shortcomings. First, if there is insufficient restraint to hold the radially compressed gasket in place, the lateral or axial force exerted on such gasket due to hydrostatic pressure can cause the gasket to slip or “blow out,” thereby causing leakage between the spigot and bell. With a radially compressed gasket, such restraint is at least partly frictional by nature, due to the inherent properties of the gasket material. However, in order to facilitate joint assembly, it is often necessary to apply a lubricant to the gasket, which lowers its coefficient of friction, thus exasperating the problem of slippage.
To compensate for this, and in order to achieve a watertight joint to a pressure of 10.8 psi, it is desirable and beneficial to minimize the lateral or axial force exerted on such a gasket. Since the axial force exerted on the gasket is directly proportional to the difference of the diameters of the bell and spigot squared (Dbell2−Dspigot2), to reduce this force, the clearances between the bell and spigot need to be kept at a minimum. By way of example, for a lateral gasket force of 200 lb, and a coefficient of friction between the gasket and pipe wall of 0.4, it is estimated that the compressive force on the gasket has to be on the order of 500 lb to restrain the gasket from slipping.
Thus, in order to minimize the potential for gasket slippage, it becomes necessary to have a very small clearance between the bell and the spigot. This minimizes the force on the gasket when the joint is pressurized, but, from a manufacturing standpoint, can be difficult to hold the tight tolerances required. Also, while closer tolerances mean better sealing, it does lead to more difficult installation, due to the friction between the gasket and the bell.
Another issue with the current bell and spigot design utilizing a radially compressed gasket is that the internal water pressure in the joint system causes the bell to expand at the point of intersection with the gasket, thereby causing further leakage. The bell has to be sufficiently stiff to restrain this force, and with larger pipe sizes (e.g. 42″ and larger), it becomes quite difficult to manufacture a bell with enough stiffness to meet this criteria. Therefore, as stated previously, some manufacturers have resorted to adding reinforcing bands around the exterior of the bell to keep the bell from expanding (and thereby creating a leak in the system). These bands can be effective, but are also costly and add to the manufacturing process.
It is evident then that the current coupling systems used for corrugated plastic pipe in watertight gravity-flow drainage and sewage applications have inherent limitations which make them more prone to leakage problems when subjected to increasing internal and external hydrostatic pressures, particularly when those pressures exceed 10 psi. The present invention, as described hereafter, marks a departure from conventional coupling systems and contemplates a new and improved coupling system that will eliminate or significantly reduce the above-mentioned problems, and others, of the current joint designs.